Not familiar with that particular model, but it’s a business laptop (which means well-built, sturdy and good components) and the specs seem pretty good. Don’t bother with Win 7, Win 8.1 is actually a better engine for a DAW, even if you may not like the GUI (come on, we musicians should be practical, not fashionistas…) Besides, Win 9 is coming in a few months and reportedly it’s a free upgrade if you have 8/8.1.
Sure, you can run NI libraries from an external USB 3.0 drive, but there are a couple of things to consider:
I don’t know if that particular model has room for an extra internal drive, but if it doesn’t, you can always remove the optical and put it there. One of the reasons I bought my 17" HP Elitebook is because it has room for 2 internal drives plus a swappable bay that can accommodate either an optical or a third drive. Sure, it’s not as “portable” as a Surface 3 Pro, but having three internal drives/ssd’s in a laptop is really something I now consider indispensable for my on-the-go work.
I would get the 512 SSD plus an internal SATA III 1tb 7200 rpm.